Carbenium ions heteroatom-stabilized

Typically, low temperatures are necessary to suppress these reactions. Additionally, other considerations, such as stabilization of the propagating centers, use of additives to suppress ion-pair dissociation and undesirable protic initiation, and the use of high purity reagents to prevent the deactivation of the carbenium by heteroatomic nucleophiles are often required. However, by careful... 

Some of the most important reaction intermediates in organic chemistry are the carbocations. Neglecting some heteroatom-stabilized cations, most carbocations are divided into two groups trivalent carbenium ions and five-coordinate or higher coordinate carbonium ions. The parent carbenium ion is CHJ, and the parent carbonium ion is CHJ. Carbonium ions have been proposed as reactive intermediates in superacid-catalyzed reactions however, they have never been directly observed in condensed media. In contrast, a variety of carbenium ions have already been prepared in superacidic media and been characterized by various physical methods, mainly 13C NMR spectroscopy (5). 

For the purposes of this review, we include probe molecules that can be either directly adsorbed or formed in situ. Examples of the latter case are carbenium ions and related electrophilic species. We will also consider several important heteroatom-substituted carbenium ions and heteroatom analogs of carbenium ions. Acylium ions are the intermediates in Friedel-Crafts acylation reactions (96). The most simple, stable acylium ion is the acetylium ion, 1, and others are formally derived by replacing the methyl group with other R groups. Oxonium ions, formed by alkylation of an ether, resemble carbenium ions but are in fact onium ions in terms of their structures. Their stabilization requires strongly acidic media, and like carbenium ions, oxonium ions have been proposed as intermediates in a... 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

Due to resonance stabilization and their higher nucleophilicity, heteroatoms stabilize the growing carbenium ions better than alkyl and aryl groups do N-vinyl carbazole is more reactive than vinyl ethers because of nitrogen s higher nucleophilicity. However, the reactivity of the growing carbenium ions follows the opposite order shown above, with the most stable... 

Substituents with a-heteroatoms such as a-alkoxy, thio, halo, and amino groups stabilize through resonance to form, for example, oxonium (R—0+=CH2) or halonium ( + Br=CR2) ions. Aryl groups, especially those with electron-donating substituents, also delocalize the positive charge through resonance, with the stability of carbenium ions increasing... 

Substituents with a-heteroatoms stabilize carbenium ions via resonance, which leads formally to oxonium ions [Eq. (3)]. 

The most reactive monomers in cationic polymerizations, Af-vinyl carba-zole, vinyl ethers, and p-methoxy-a-methylstyrene provide the most stable carbenium ions. Carbenium ions generated from the first two monomers are stabilized by a-heteroatoms, whereas the latter monomer generates a tertiary carbenium ion with a strongly electron donating p-substituent. Carbenium ions with two alkoxy groups, such as 1,3-dioxo-lane-2-ylium cations and their acyclic analogs (Chapter 6) are stable at room temperature in the absence of moisture. 

Griitzmacher, H. and Marchand, C.M. (1997) Heteroatom stabilized carbenium ions. Coord. Chem. Rev., 163, 287-344. 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

The effects of substituents at the 1-, 2-, 3-, and 7-positions of 7-methylcyclohepta-triene on the cycloheptatriene-norcaradiene equilibrimn have been examined. Where the C-7 substituent is a carbenium ion stabilized by heteroatoms, e.g. the bis(methylamino) substituted cation (255), the equilibrium is towards cycloheptatriene ... 

Another case in which the different characteristics of a group need to be considered is the effect of heteroatoms. When they are not directly attached to the carbon undergoing ionization, an electronegative atom such as O, N, or S destabilizes carbenium ions and slows SnI reactions due to inductive electron withdrawal. However, when they are directly attached, they accelerate SnI reactions due to resonance stabilization. Nitrogen is the best at stabilizing a carbenium ion and thereby accelerating SnI processes. Sulfur seems to have a variable effect in this resonance stabilization its orbitals are larger than those of C and hence the sizes are mismatched, but S is very polarizable. 

Heteroatoms are not the only groups that will facilitate an SnI reaction when in proximity to the cationic center. Resonance effects in allyl or benzyl carbenium ions stabilize the cationic center and hence facilitate the substitution reaction. Yet, n systems further away from the cationic center can also get involved if their geometry is such that they are oriented toward the carbenium ion s empty p orbital. Consider the solvolysis of cholesterol tosylate in Figure 11.7. Two products are formed, and the solvolysis rate is approximately 100 times... 

As described in Section 11.5.12 with reference to SnI reactions, a hydride or alkyl shift in a carbenium ion is a common rearrangement. It entails the movement of a hydrogen, alkyl, or vinyl / aryl group from a carbon adjacent to a carbenium ion to the electrophilic center in order to create a more stable carbenium ion. Many rearrangements have a similar migration as a key step, with the additional feature of a heteroatom on the p-carbon that stabilizes the newly formed electron deficient center (Eq. 11.46). Two prototype examples are the pinacol rearrangement and the benzilic acid rearrangement. 

Based on the analysis of the electronic structure of vinylidene intermediates, we wanted to test whether allenylidene complexes of gold(I) could be prepared. First, the gold acetylides 49 were prepared then an alkylation delivered 50 (Scheme 4.20). A detailed structure investigation, based on experimental and computational methods, followed [43]. The strong donors, which allow 50 to be isolated, stabilize the carbenium ion with the lone pair of the heteroatom (O or N). Thus no significant Au-C double bond character could be detected. 

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